1. Field of the Invention
This invention relates to an ultrasonic transmitter-receiver (hereinafter referred to as "transreceiver"), and more particularly to an ultrasonic transreceiver for measuring the flow velocity and flow rate of gas flowing through a pipe using ultrasonic wave.
2. Description of the Prior Art
An apparatus has been developed and extensively used in a variety of fields which is adapted to measure the flow velocity and flow rate of gas such as air, steam, exhaust gas or the like flowing through a pipe utilizing a variation of propagation velocity of ultrasonic wave propagated through the gas.
Such a conventional apparatus for measuring the flow velocity and flow rate of gas utilizing ultrasonic wave is typically constructed in such a manner as shown in FIG. 1. More particularly, an apparatus shown in FIG. 1 includes a pair of ultrasonic transreceivers 10A and 10B arranged on a wall of a pipe 12 in a manner to be obliquely opposite to each other. The ultrasonic transreceivers 10 are alternately excited and controlled by a controller 14. Thus, in the apparatus, the flow velocity and flow rate of gas flowing through the pipe 12 are measured on the basis of the difference between a period of time for which ultrasonic wave is propagated from the transreceiver 10A to the transreceiver 10B and that for which it is propagated from the transreceiver 10B to the transreceiver 10A.
In general, the conventional ultrasonic transreceivers 10A and 10B used in the flow velocity and flow rate measuring apparatus each are typically constructed as shown in FIG. 2. The conventional ultrasonic transreceiver 10 includes a housing 16 formed of a metal material to have an upper open end and a lower closed end. The ultrasonic transreceiver 10 also includes a piezoelectric transducer 18 formed of a ceramic material and having piezoelectric characteristics, which is fixed on a bottom wall of the housing 16 by means of an adhesive. The piezoelectric transducer 18 is formed on an upper surface thereof with a first electrode or positive electrode 20. The positive electrode 20 is made of a suitable material such as gold, silver, nickel or the like into a film-like shape by vapor deposition, baking, plating or the like. Also, the piezoelectric transducer 18 is formed on a lower end thereof and a peripheral surface in proximity to the lower end with a second electrode or negative electrode 22 and a connection 24 in a manner to be contiguous to each other. The negative electrode 22 is formed in substantially same manner as the positive electrode 20.
The ultrasonic transreceiver 10 also includes an insulating member 26 which is formed with a vertical through-hole and securely fitted in the upper open end of the housing 16, and a pipe-like plug 28 of which a lower portion is fixedly fitted in the central through-hole of the insulating member 26. The plug 28 and positive electrode 20 are electrically connected to each other by means of a first lead wire 30. The connection of the lead wire 30 with respect to the plug 28 and positive electrode 20 is carried out by soldering. Likewise, the negative electrode 22 and housing 16 are electrically connected to each other through a second lead wire 32 by soldering.
The ultrasonic transreceivers 10A and 10B constructed as described above, as shown in FIG. 1, are integrally mounted in terminal boxes 34A and 34B which are then fixed in sockets 36A and 36B of the pipe 12 by means of screws or flanges, respectively.
Unfortunately, the measuring of flow velocity and flow rate of hot gas such as steam or the like flowing through the pipe 12 by means of the ultrasonic transreceivers 10A and 10B described above causes the transreceivers to be contacted directly with the hot gas, so that the transreceivers may undergo a sudden temperature variation and a severe heat cycle.
Such temperature variation and heat cycle lead to the generation of deviation thermal strain due to the difference in a thermal expansion coefficient between the housing 16 and the piezoelectric transducer 18, resulting in internal stress being produced at the adhesive-bonded portion between the housing 16 and the piezoelectric transducer 18. When the so-produced internal stress or thermal stress is above the bond strength of an adhesive used in the transreceiver 10, any breakage occurs at the adhesive-bonded portion to cause the piezoelectric transducer to be gradually peeled off from the housing 16.
Such peeling causes a gas boundary layer to be formed between the piezoelectric transducer 18 and the housing 16, so that an ultrasonic pulse signal received by the piezoelectric transducer 18 may be subjected to the attenuation of propagation due to diffusion, absorption, dispersion and the like to be highly decreased in level. It would be considered to use an adhesive of high bond strength in order to prevent such peeling. However, this causes the internal stress due to the deviation thermal strain to be generated in the interior of the piezoelectric transducer 18, so that it may be broken to a degree sufficient to fail in the transmitting and receiving of ultrasonic wave. Accordingly, the transreceiver fails to measure the flow velocity and flow rate of gas flowing through the pipe 12.
Further, the direct contacting of the transreceiver with hot gas causes it to be heated to a temperature approximate to that of the hot gas. This leads to the melting of solder used for connecting the lead wires 30 and 32 to the electrodes 20 and 22 of the piezoelectric transducer 18, resulting in a failure in electrical connection therebetween.
In order to minimize such a problem, it would be proposed to use high temperature solder. However, this promotes a solid metal-molten metal diffusion phenomenon which causes metal forming the electrodes 20 and 22 to be diffused into molten solder metal during soldering, because the soldering is carried out at a high temperature. This results in the peel strength of the electrodes 20 and 22 being highly decreased to lead peeling of the lead wires 30 and 32 from electrodes.
Also, the exposing of the piezoelectric transducer 18 to a high temperature for a long period of time highly promotes a solid metal-solid metal diffusion phenomenon which causes metal of the electrodes 20 and 22 to be diffused into solder metal, so that the electrodes may be peeled from the piezoelectric transducer 18 together with the lead wires 30 and 32.
Such a failure in electrical connection between the electrodes and the piezoelectric transducer due to the peeling of the electrodes therefrom renders the measuring of the flow velocity and flow rate of gas flowing through the pipe substantially impossible. This requires the ultrasonic transreceiver to be frequently replaced.
Accordingly, it would be high desirable to develop an ultrasonic transreceiver which is capable of effectively preventing the formation of a gas boundary layer between a piezoelectric transducer and a housing due to thermal stress and the breakage of the piezoelectric transducer and preventing the peeling of electrodes from a piezoelectric transducer even when it is exposed to a high temperature.